It is known that a polymer having a crosslinkable functional group at a terminus of its main chain cures singly or in combination with a suitable curing agent to give a cured product with high heat resistance and durability. Among them, a polymer having an alkenyl or crosslinkable silyl group at a terminus of its main chain is its representative example. A polymer having an alkenyl group at a terminus of its main chain crosslinks and cures in the presence of a hydrosilyl-containing compound as a curing agent or by way of photopolymerization reaction. Moreover, a polymer having a crosslinkable silyl group at a terminus of its main chain absorbs moisture in the presence of a suitable condensation catalyst to provide a cured product.
The main chain of such a polymer having an alkenyl group or a crosslinkable silyl group at a terminus of its main chain includes polyether polymers such as polyethylene oxide, polypropylene oxide and polytetramethylene oxide; hydrocarbon polymers such as polybutadiene, polyisoprene, polychloroprene, polyisobutylene, etc., inclusive of hydrogenation products thereof; and polyester polymers such as polyethylene terephthalate, polybutylene terephthalate, polycaprolactone, etc. According to the main chain structure and mode of crosslinking, those polymers have been used in various applications.
These polymers can be obtained by ionic polymerization or polycondensation reaction, vinyl polymers having crosslinkable functional groups at a terminus of its main chain as synthesized by radical polymerization have little been used in commercial applications. Among vinyl polymers, (meth)acryl polymers have high weather resistance, good clarity and other favorable characteristics, which are not found in polyether polymers, hydrocarbon polymers, and polyester polymers mentioned above. For example, those polymers having alkenyl or crosslinkable silyl groups not in their main chain but in their side chains have been utilized as coating agents featuring high weather resistance.
As compared with this vinyl polymer having crosslinkable functional groups in its side chain, a vinyl polymer having crosslinkable functional groups at a terminus of its main chain could provide cured products with excellent properties about curing. Accordingly, many workers have been endeavoring to develop a expedient technology for preparing such a polymer in earnest but it is not an easy task to establish a commercially useful production technology.
Japanese Kokai Publication Hei-1-247403 discloses a method for synthesizing a vinyl polymer having an alkenyl group at both termini, which comprises using an alkenyl-containing disulfide as a chain transfer agent. Japanese Kokai Publication Hei-6-211922 discloses a method of synthesizing a vinyl polymer having an alkenyl group at both termini, which comprises synthesizing a vinyl polymer having a hydroxyl group at both termini by means of a hydroxyl-containing disulfide as a chain transfer agent, and synthesizing a vinyl polymer having an alkenyl group at both termini by exploiting the reactivity of the hydroxyl groups. However, these methods require to use a chain transfer agent in a substantial amount in order to introduce alkenyl groups into both termini without fail and this entails difficulties in process control. Moreover, since the vinyl polymers as obtained by these methods have a bond between an alkenyl group and its main chain via a sulfur atom, the polymers have a reverse effect on their weather resistance, thus their inherent property, and further give a problem in odds of sulfur itself.
In the otherhand, Japanese Kokai Publication Sho-59-168014 discloses a method of introducing a crosslinkable silyl group to both termini of a vinyl polymer, which comprises using a crosslinkable silyl-containing disulfide as a chain transfer agent. Moreover, Japanese Kokai Publication Sho-61-133201 discloses a method of introducing a crosslinkable silyl group to both termini of a vinyl polymer, which comprises using hydrosilane or silane halide each having a crosslinkable silyl group. However, in these methods, it is difficult to certainly introducing a crosslinkable silyl group to both termini thereof so that cured products having satisfied properties cannot be obtained. Further, since the ordinary radical polymerization reaction is used in these methods, the molecular weight and molecular weight distribution (the ratio of weight average molecular weight to number average molecular weight) of the product polymer cannot be easily controlled.